Something in the function tutorial confused me.

[greg]

Steve Holden wrote:
> OK. The difference is that [] is a mutable value, while None is 
> immutable.

No, it's not. It has nothing to do with mutability
vs immutability.

The difference is that in the first version the
expression [] is evaluated only *once*, when the
function is defined. Therefore there is just one
list object getting re-used.

In the second version, the expression [] gets
evaluated *each* time the function is called,
creating a new list object each time.

> When the function starts out with None as y's value any assignment to y 
> causes it to reference a different value  (since None is immutable).

You're confused. Assigning to y simply causes it
to reference whatever object is being assigned.
This is always true, regardless of what y was
referencing before.

To see that the immutability of None has nothing
to do with it, try the following version:

def f(x, y = []):
   y = []
   y.append(x)
   print y

f(17)
f(42)

Try to work out what it will do, then try it, and
see if you understand why it does what it does.

--
Greg